The tub needs a trap.
I live in a 1950's brick rancher and we've noticed a sulpher/dead smell that comes and goes. I pin pointed it down to the basement level bathtub area. So I tore out the wall half expecting an armadillo or mole nest, but instead noticed the tub and toilet drain going down through the slab at this location, but no "p" trap on the bathtub drain. We have a septic tank because we are below graid for city sewer, so do you think it's just the septic smell coming up the tub drain? Has anyone encountered this, what was it and what did you do about it?
Tuba/shower P traps are usually under the slab so it's a bit tricky to see. Take the strainer cover off of the drain and use a flashlight to look down. If there is a trap, you should see water. If there is no trap (pretty unlikely since all drains are supposed to have a P trap) then you have a problem that is probably beyond DIY. The tub will have to be removed, the slab cut open, and a trap installed. It might very well be a can of worms that a DIY would be in deep doodoo attempting.